Frequency-separator waveguide module with double circular polarization

ABSTRACT

The module comprises an input/output access point at a first end of a waveguide with a square cross section, called a square waveguide, two access points made of waveguides with a rectangular cross section, called rectangular waveguides, placed side by side at a second end of the square waveguide and a septum positioned in the square waveguide at the end of a separation region common to the two rectangular waveguides in order to allow the production of two circular polarizations of opposite handedness each associated with a rectangular waveguide. The module is arranged so as to form a diplexer in which the septum is included and where the access points by rectangular waveguide are extended by filters, each access point being endowed with a filter provided in order to transmit a frequency band which is different. The steps of the septum are dimensioned so as to compensate for reflections.

The invention relates to a frequency-separator waveguide module withdouble circular polarization more particularly intended to serve as anantenna access module for a transmitter-receiver operatingsimultaneously in two frequency bands and with circular polarizationswhich are opposite for transmission and for reception.

This type of transmitter-receiver and, consequently, this type of moduleare especially intended to be used in systems transmitting and receivingat high bit rates via low-orbit satellites. The possibility ofsimultaneous transmission and reception with the same access point to asystem means that it is possible to obtain high isolation between thetransmission path and the reception path, at the antenna access point,and double circular polarization with a high degree of purity ofpolarization over a large frequency band. Right circular polarizationfor the transmission path and left circular polarization for thereception path are, for example, chosen. Cross-polarization of less than−25 dB, corresponding to an axial ratio of less than 1 dB, at thetransmission access point and at the reception access point is, forexample, sought.

A conventional approach for obtaining circular polarization from alinearly polarized field is shown diagrammatically in FIG. 1. Saidapproach combines an exciter 1 with a polarizer 2 made using waveguidetechnology. The exciter 1 separates a frequency band Tx used intransmission and a frequency band Rx used in reception. The polarizer 2generates circular polarization, the handedness of which depends on theorientation of the electric field vector, as symbolized by the labelsRCP and LCP, one assumed to correspond to right polarization and theother to left polarization.

A known waveguide component which makes it possible to produce suchcircular polarizations is a system with a central septum where stepsproduced on the septum border create a horizontal field which recombineswith a vertical input field in order to produce circular polarization.In a known embodiment, shown schematically in FIG. 2, the polarizer 2comprises two access points 3A, 3B made of waveguide with a rectangularcross section, symmetrically arranged with respect to a central plane ofline XX′, which join each other at an end which is extended by a septum4, in order to open out into a waveguide portion 5 with a square crosssection where the septum is placed. The right or left circularpolarization is obtained by the progressive creation of a horizontalelectrical field vector, by the steps on the plate forming the septum 4and the recombination of this horizontal vector with the vertical vectorcorresponding to the linear polarization of the access point 3A or 3Bfrom which it comes. The two access points 3A and 3B therefore make itpossible to produce two circular polarizations having orientations whichare opposite for two different frequency bands at the access point 3Cwhich constitutes the end of the portion 5 with a square cross section.The latter may possibly be fitted with a normal transition (not shown),making it possible to pass from a square section to a circular section,if necessary.

The separator 1 is combined with the polarizer 2 in order to separatethe transmission Tx and reception Rx paths for each of the access points3A and 3B. Provision is made to absorb, via a load, the band which isnot useful at each of these access points 3A, 3B.

This is because, if the access points 3A and 3B are used alone, withouta separator as envisaged above, there is a reflection of the frequencyband which is not used at one access point, that is therefore of theband used for reception in the case of an access point used intransmission and vice versa. The consequence of these reflections in thedirection of the septum results in mismatching of the polarizer. This isthe reason for inserting a load, in this case assumed to be 50 ohms, inone arm and, for example, in an arm 6A parallel to the arm 7A at theaccess point 3A where the arm 7A is used for transmission, and thereason for inserting a similar load in the arm 6B parallel to the arm 7Bat the access point 3B where the arm 7B is used for reception.

However, this solution has the drawback of being bulky because of theuse of a separator with multiple arms for access. Furthermore, it isexpensive since the components employed, such as the filters, thetransitions and the septum, are awkward to produce and assemble.

The invention therefore provides a frequency-separator waveguide modulewith double circular polarization more particularly intended to act asan antenna access module for a transmitter-receiver operatingsimultaneously in two frequency bands and with polarizations which areopposite for transmission and in reception.

The frequency-separator waveguide module comprises input/output accesspoint to a first end of a waveguide with a square cross section, calleda square waveguide, two access points made of waveguides with arectangular cross section, called rectangular waveguides, placed side byside at a second end of the square waveguide and a septum positioned inthis square waveguide at the end of a central separation region commonto the two rectangular waveguides in order to allow the production oftwo circular polarizations of opposite handedness each associated withone of the rectangular waveguides.

According to one feature of the invention, the module is arranged so asto form a diplexer in which the septum is included and where the accesspoints by rectangular waveguide are extended by filters, each accesspoint being endowed with a filter provided in order to transmit afrequency band which is different, the steps of the septum beingdimensioned so as to compensate for the reflections of the frequenciesrespectively rejected by each filter towards the said septum.

The invention also provides a transmitter-receiver for operatingsimultaneously in two frequency bands and with circular polarizationswhich are opposite for transmission and for reception.

According to one characteristic of the invention, thistransmitter-receiver comprises an antenna access module consisting of awaveguide module as defined above.

The invention, its features and its advantages are specified in thefollowing description in connection with the figures mentioned below.

FIG. 1 shows an outline diagram of a waveguide device according to theprior art making it possible to obtain circular polarization from alinearly polarized field.

FIG. 2 shows a schematic view relating to a known waveguide module foraccess to an antenna.

FIG. 3 shows a schematic view relating to a waveguide module for accessaccording to the invention.

FIG. 4 shows a perspective view relating to an alternative embodiment ofan access module according to the invention.

FIG. 5 shows a diagram representing performances likely to be obtainedwith a septum according to the prior art, within the context of anaccess module with no filter at the two rectangular access points.

FIGS. 6 and 7 show diagrams representing performances obtained beforeoptimization showing the perturbations introduced, when the septum iscombined with filters located in the extension of the rectangular accesspoints within the context of a module according to the invention.

FIGS. 8 and 9 show diagrams representing performances more particularlyobtained, before optimization, at the transmission and reception bandstaken by way of example, with the filterless septum envisaged above.

FIGS. 10 and 11 show enlarged diagrams relating to the performances moreparticularly obtained, after optimization, for the transmission andreception bands taken by way of example, with the septum fitted withfilters.

A frequency-separator waveguide module with double circularpolarization, according to the invention, is shown schematically in FIG.3. The module includes a diplexer 8 in which a septum 9 with multiplesteps is positioned, which septum is used as a polarizer. This septum ishoused inside a waveguide portion 10 with a square cross section, hereshown in dashed lines. The diplexer has two access points 11A and 11Bconsisting of short waveguide elements which are parallel and which havea rectangular cross section, one of them, such as the access point 11A,being intended to be used in transmission and the other, such as theaccess point 11B, in reception. The waveguide elements with arectangular cross section corresponding to these access points 11A, 11Bare connected to the waveguide portion 10 on each side of a central andcommon separation region 12 penetrating the waveguide portion 10 at oneend. In the proposed exemplary embodiment, the septum 9 consists of athin plate with steps which has a base positioned at the end of theseparation region 12 inside the waveguide portion 10. The steps, whichit has laterally and which reduce it from its base towards its apex, liein a first part of this waveguide portion. Moreover, the diplexercomprises a square access point 11C which opens at the end of thewaveguide portion 10 which is away from the end where the tworectangular access points 11A and 11B open. These two access points areeach provided for a particular frequency band which is different. Thisstructure is used to obtain a module with a dual-band septum. To thisend, the two access points 11A and 11B, which are completely independentfrom each other, are respectively equipped to allow each to filter oneof the two frequency bands.

Filtering at a high frequency band may be carried out naturally byreducing the cross section at a rectangular access point in theextension of this access point, as shown diagrammatically by thereducing element 13A forming a filter for the access point 11A in FIG.3. The cut-off frequency is changed to prevent the propagation of lowfrequencies.

Filtering at a low frequency band is carried out at the otherrectangular access point, here it is assumed to be obtained bypositioning transverse metal inserts or “stubs” in a portion located inthe extension of this access point, as symbolized by the inserts 14Bplaced internally on each side of the rectangular waveguide portionrelative to the access point 11B.

A significant saving with regard to overall size is obtained for amodule according to the invention if this module is compared with amodule according to the prior art having a separator with four arms, asdescribed in relation to FIG. 2. This facilitates integrating the moduleaccording to the invention in an assembly where it is needed, and inparticular as an access circuit for an antenna in the case of atransmitter-receiver as envisaged above.

The solution proposed in connection with FIG. 3 is not unique and, inparticular for reasons of compactness and of simplifying the mechanicalproduction of the module, a solution as shown diagrammatically in FIG. 4is provided.

The module shown in this FIG. 4 consists of a diplexer 8′ similar to thediplexer 8 shown in FIG. 3. This diplexer 8′ identically comprises awaveguide portion 10′ with a square cross section where a septum 9′ isplaced. The diplexer 8′ has two access points, with a rectangular crosssection, 11A′ and 11B′ placed side by side, like the access points 11Aand 11B of the diplexer 8. One of these rectangular access points, inthis case 11A′, is extended by a reducing element of cross section 13A′,which is constructed like the access point 11A and which also allowsfiltering at a high frequency band. The other rectangular access point,in this case 11B′, is equipped to filter at a low frequency band andhere it is extended by a portion where transverse metal inserts 14B′ aremade externally. In the proposed example, these inserts 14B′ are made inthe form of transverse grooves opening towards the inside of therectangular waveguide portion where they are made on at least one of therectangular and flat wall parts which laterally define this waveguideportion. In the proposed embodiment, the grooves are made in regionswhich project outwards from the volume from that flat wall part which isoutermost. A mechanical embodiment which is particularly simple toimplement may therefore be obtained.

Whichever of the solutions according to the invention is chosen, thefact remains that the filtering carried out by means positioned in theextension of the rectangular access points of the module tend tointroduce perturbations in the transmission coefficients of this module,with respect to those which would be obtained by means of the septumused without filters.

A waveguide module according to the invention intended for atransmitter-receiver, transmittering in a frequency band Tx extendingfrom 14 to 14.5 GHz and receiving in a band Rx extending from 11.7 to12.7 GHz is presupposed. Moreover, it is presupposed that there is aneed to have an axial cross polarization greater than −25 dB and aninsulation greater than 20 dB in the transmission and reception bands.

The septum provided in the module conditions the quality of insulationobtained to the extent that the latter depends directly on thediscriminating power of the cross polarization.

A polarizer with a septum having a band extending from 11.7 to 14.5 GHzis assumed to be chosen, as it is known that its bandwidth is a functionof the number of steps which the thin plate of which it is composed hasand that it is possible to obtain an axial ratio of about 0.6 dB for thefrequency band envisaged above with a septum having four steps.

Assuming rectangular access points, made using waveguides in the WR75standard of, for example, 19.05 by 9.525 mm, and a square waveguide of20 by 20 mm, it is possible to obtain a good match with the envisagedbandwidth, the cut-off frequency for the TE10 transverse electrical modebeing 7.49 GHz. Furthermore, the TE20 transverse electrical mode is notlikely to be excited since its cut-off frequency is 14.99 GHz.

The step length is about a quarter of the guided wavelength λg, whichcorresponds to 6.97 mm at the central frequency of 13.1 GHz and whichleads to a septum plate length of about 35 mm.

As is known, the quality of the excitation depends on the position ofthe exciting probe with respect to the short-circuit end of the guidewhere it acts and this position corresponds to a movement of the probeaway from this end by about a quarter wavelength λg. Here, the septum isassumed to be placed at a distance from the probe of about λg, so thatit is possible to drive the septum in the fundamental mode.

To obtain good quality circular polarization, the phases of theorthogonal modes present in the square waveguide are shifted by 90° andhave the same amplitude so as to have transfer coefficient values S13and S23 of 3 dB for each of the modes exploited. S13 corresponds to thetransfer coefficient between ports 1 and 3 and S23 to the transfercoefficient between ports 2 and 3, the ports 1, 2 and 3 correspondingrespectively to the access points 11B, 11A and 11C of FIG. 3. Moreover,the modes 1 and 2 correspond respectively to a vertical orientation ofthe electrical field and to a horizontal orientation of this field.

The diagram presented in FIG. 5 illustrates the performance obtainedwith a septum having four steps, according to the prior art, provided ina module according to the invention and as defined above, withoutfilters at the two rectangular access points of the module.

The width of the frequency band involved is from 11.5 to 14.5 GHz, asshown on the X-axis, a graduation of 0 to −60 dB being provided on theY-axis. The performance is virtually identical for the transfercoefficients S13 and S23 in mode 1, as shown diagrammatically by avirtually horizontal curve 1. This is virtually the same for thetransfer coefficients S13 and S23 in mode 2, as shown diagrammaticallyby a curve 11 which dips slights in the vicinity of the frequencies 12.5and 13.5 GHz and which has a negative peak reaching more than −10 dB inthe vicinity of 13.6 GHz frequency. Modes 1 and 2 correspondrespectively to the vertical and horizontal polarizations of theelectrical field.

Curves 1 and 11 show that the limit of 3 dB is held for frequenciesbetween 11.8 and 14.3 GHz and therefore for the entire receivingfrequency band, in contrast this limit is not held for all thefrequencies of the transmission band and in particular in the vicinityof the 13.6 GHz frequency, already mentioned above. Provision istherefore made to optimize performance at this level.

The diagrams presented in FIGS. 6 and 7 show the perturbations which arecaused by the presence of the filters placed in the extension of therectangular access points, each for purposes of selectively eliminatingthe frequency band which is not transmitted by the access point inquestion, as indicated above.

Curves III and IV presented in FIG. 6 show the respective performanceobtained for the coefficient S23 in mode 1 and 2. The curve III relatingto the coefficient S23 in mode 1 is virtually coincident with the curveIV for the range of frequencies going from 11.5 GHz to 13.5 GHz with theexception of a region located in the vicinity of the frequency 12.1 GHzwhere the curve III has a peak going up to about −36 dB and where thecurve IV has a peak going down to −59 dB. The two curves separateespecially around the frequency 13.65 GHz where the curve IV has a peakgoing down to −12 dB while the curve III has a peak going up to −3 dB.The parts of curve III and IV which are located in the frequency bandroughly between 13.7 and 14.5 GHz, within which the frequency band Tx of14 to 14.5 GHz exploited in transmission is found, are enlarged in FIG.8 for this band. The curve II, relating to the transfer coefficient S23in mode 1, is between −1 and −3 dB for a frequency band ranging from13.7 to 14.4 GHz and the curve IV, relating to the transfer coefficientS23 in mode 2, is between −4 and −7 dB for a frequency band ranging from13.7 to 14.5 GHz. Such a module does not allow the desired performanceto be obtained. The invention aims to act on the construction of theseptum in order to compensate for the perturbations, created in thetransmission band, by readjustment of the steps which the thin plateforming the septum has, by modifying, by trial and error, the length andthe depth of the various steps.

The curves V and VI presented in FIG. 7 show the respective performanceobtained for the coefficient S13 in mode 1 and in mode 2 in a frequencyband extending from 11.5 to 15 GHz.

The curves V and VI are in a region between −2 and −5 dB between thefrequencies of 11.5 and 12.7 GHz, where the frequency band Rx exploitedin reception is located, with the exception of a limited region,virtually centred on the frequency 12.1 GHz, where the two curves show adownward peak. FIG. 9 corresponds to an enlargement of the parts ofcurves V and VI between the limiting frequencies of 11.7 and 12.5 GHz ofthe receiving band.

A low point at more than −10 dB is noticed for the curve V, relating tothe coefficient S13 in mode 1, with a lower point of −19 dB for thecurve VI relating to the coefficient S13 in mode 2 (FIG. 7).

In a module according to the invention, these perturbations, which arecaused by the filtering and which affect the transmission coefficients,are compensated for by a dimensional readjustment of the steps of theseptum. This readjustment is carried out in steps until an optimumresult, which is illustrated here in FIGS. 10 and 11, is obtained. Thecurves III′, IV′, V′ and VI′ presented in these figures showrespectively the variations of the coefficients S23 in mode 1 and 2 andS13 in mode 1 and 2 measured in dB and given as a function of thefrequency, after optimization, for the envisaged module according to theinvention. The reduction of the negative peaks presented by the curvesV′ and VI′ in FIG. 11 compared to the corresponding curves V and VI inFIG. 9 should be noted in particular.

If, for example, equality of amplitude for the transmitted orthogonalmodes is chosen as an optimization factor for each access point, it maybe translated in the form of the following criteria:

S13 mode 1=S13 mode 2=−3 dB over the 11.7 to 12 GHz band S23 mode 1=S23mode 3=−3 dB over the 13.9 to 14.1 GHz band.

Improving the performance over the optimized bands more particularlyresults in the values obtained from the curves presented above whichappear in the table given below by way of example.

Considering the septum with four steps envisaged above, which is assumedto have a base of 20 mm and four steps whose width is respectively 15.69mm, 9.62 mm, 5.67 mm and 2.56 mm, an optimized septum is proposed herehaving the same base as before and four steps whose widths arerespectively 16.79 mm, 9.32 mm, 6.71 mm and 2.58 mm.

According to the table mentioned above, the following is obtained:before after optimization optimization S13 mode 1-S13 mode 2 to 11.7 GHz  3 dB 1.6 dB   to 12 GHz 1.7 dB 1.3 dB S23 mode 1-S23 mode 2 to 13.9GHz 3.2 dB 1.3 dB to 14.1 GHz 5.6 dB 2.6 dB

A difference of 1.3 dB between the amplitudes, with a phase shift ofbetween 84 and 90°, leads to an axial ratio better than 1.75 dB.

Insofar as the phase has not been taken into account within the contextof this optimization, it is possible to carry out an additionaladjustment by changing the length of the steps of the septum.

Modifying the width of the septum steps makes it possible to compensatefor the defects caused by the filters placed in the extension of therectangular access points. Dimensioning these steps makes it possible tocompensate for the reflections of the frequencies which are respectivelyrejected by each filter towards the septum. The optimization is, forexample, carried out by trial and error by varying the size of the stepsand by producing simulations for each variation.

The polarizer with a dual-band septum which is obtained makes itpossible to produce a frequency-separator waveguide module with doublecircular polarization. This module is more particularly intended to actas a link between an antenna and a transmitter-receiver intended tooperate simultaneously in two frequency bands with circularpolarizations which are opposite for transmission and for reception. Thetransmitter is connected to one of the rectangular access points which,in this case, is assumed to be the access point 11A, or 11A′, equippedwith a reducing element 13A or 13A′, if the transmitting frequency bandis higher than that of reception, as envisaged here. The receiver isconnected to the other rectangular access point and the antenna isconnected to the access point located at the other end of the squarewaveguide portion 10 or 10′.

1. Frequency-separator waveguide module, comprising an input/outputaccess point at a first end of a waveguide with a square cross section,called a square waveguide, two access points made of waveguides with arectangular cross section, called rectangular waveguides, placed side byside at a second end of the square waveguide, and a septum positioned inthis square waveguide at the end of a central separation region commonto the two rectangular waveguides in order to allow the production oftwo circular polarizations of opposite handedness each associated withone of the rectangular waveguides, wherein said module is arranged so asto form a diplexer in which the septum is included and where the accesspoints by rectangular waveguide are extended by filters each accesspoint being endowed with a filter provided in order to transmit afrequency band which is different, the steps of the septum beingdimensioned so as to compensate for the reflections of the frequenciesrespectively rejected by each filter towards the said septum.
 2. Moduleaccording to claim 1, in which one of the rectangular access pointfilters consists of an element providing natural filtering by one ormore reductions of cross section, for the access point by rectangularwaveguide in the extension of which it is located.
 3. Module accordingto claim 1, in which one of the filters for access by rectangularwaveguide is constructed with the help of transverse metal insertsplaced internally on each side of a portion which extends the waveguidewith a rectangular cross section of this access point
 4. Moduleaccording to either of claims 1 and 2, claim 1, in which one of thefilters for access by rectangular waveguide is constructed with the helpof inserts constructed in the form of transverse grooves opening towardsthe inside of the rectangular waveguide portion in which they areproduced on at least one of the rectangular wall parts which laterallydefine this rectangular waveguide portion.
 5. Transmitter-receiverdesigned to operate simultaneously in two frequency bands and withcircular polarizations which are opposite for transmission and forreception, said transmitter-receiver comprising an antenna access moduleconsisting of a waveguide module that comprises an input/output accesspoint at a first end of a waveguide with a square cross section, calleda square waveguide, two access points made of waveguides with arectangular cross section, called rectangular waveguides, placed side byside at a second end of the square waveguide, and a septum positioned inthis square waveguide at the end of a central separation region commonto the two rectangular waveguides in order to allow the production oftwo circular polarizations of opposite handedness each associated withone of the rectangular waveguides, wherein said module is arranged so asto form a diplexer in which the septum is included and where the accesspoints by rectangular waveguide are extended by filters, each accesspoint being endowed with a filter provided in order to transmit afrequency band which is different, the steps of the septum beingdimensioned so as to compensate for the reflections of the frequenciesrespectively rejected by each filter towards the said septum. 6.Transmitter-receiver according to claim 5, in which one of therectangular access point filters consists of an element providingnatural filtering by one or more reductions of cross section, for theaccess point by rectangular waveguide in the extension of which it islocated.
 7. Transmitter-receiver according to claim 5, in which one ofthe filters for access by rectangular waveguide is constructed with thehelp of transverse metal inserts placed internally on each side of aportion which extends the waveguide with a rectangular cross section ofthis access point.
 8. Transmitter-receiver according to claim 5, inwhich one of the filters for access by rectangular waveguide isconstructed with the help of inserts constructed in the form oftransverse grooves opening towards the inside of the rectangularwaveguide portion in which they are produced on at least one of therectangular wall parts which laterally define this rectangular waveguideportion.